fix: Go 1.16 set GOFLAGS to allow change of go.mod and go.sum

[sriram-mv]

Which issue(s) does this change fix?

• Go integration tests

Why is this change necessary?

• Go 1.16 introduced a breaking change with go build no longer being allowed to change/create go.mod and go.sum files by default.
• Initial Assessment was that setting go env will not fix this, but that been set incorrectly without an "=" sign.

How does it address the issue?

• Set a go env variable to allow for changes.

What side effects does this change have?

• None, allows for build tests to start passing again.

Checklist

• Add input/output type hints to new functions/methods
• Write design document (Do I need to write a design document?)
• Write unit tests
• Write/update functional tests
• Write/update integration tests
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[aahung]

Thanks for fixing. We also need to do the same for Canary

[sriram-mv]

@aahung : https://github.com/aws/aws-sam-cli/blob/develop/appveyor.yml#L91 (There looks to be a version lock for go)